Modulated light communication system



155-518 AU 233 EX PIP-8106 x2 2,623,165 X50 p Dec. 23, 1952 H. MUELLERETAL 2,623,165

uonuumn LIGHT couuumcmxon sys'rsu i Filed Jan. 7, 1948 2 1 27 SCREEN lMAGE BRIGH T TRANSPARENT MEDIUM /3 /5 P/EZO-ELECTR/G 3 an YSTAL 3 l3TRANSPARENT 3 22 LENS MED/UM n A LS I? Q3 L/GHT 5 7 j i L 5- 25 AMHJF/ERSal/R65 P/EZ E i ELECTRIC CRYSTAL MODULA TOR OSGILLA r01? g 7 (28 za 2vF lg. 6'.

TRANSPARENT MEDIUM 1 AMPLIFIER I I 0-.- 5? 9 Fig. .9.

SGILLATOR OSCILLA TOR 'I'II WW I MODULATOR moouu ran in van tors I LHans Muel ler F198. Ruben H. R/nes Attorney Dec. 23, 1952 H. MUELLERETAL HODULATED ucm COMMUNICATION SYSTEM Filed Jan. 7, 1948 FIG.3.

FIG.4.

INVE N To s HANS MUELLE R065 BY 00 gr RINES ATTORNEY Patented Dec. 23,1952 OFFICE MODULATED LIGHT COMIWUNICATION SYSTEM Hans Mueller, Belmont,and Robert H. Rimes. Brookline, Mass.

Application January 7, 1948, Serial No. 1,002

25 Claims. 1

The present invention relates to communication. and more particularly tothe transmission and reception of signal intelligence.

An object of the invention is to provide a new and improved system forsecret signaling.

Another object is to provide a new and improved system for communicationemploying light modulation.

A further object is to provide a new and improved system fortransmitting.

A further object still is to provide a new and improved system forreceiving.

Still another object is to provide a new improved system fortransmitting and receiving.

In a copending application. serial No. 1,003, filed of even dateherewith by Robert H. Rines. there is disclosed a novel light-modulationmethod and system operable to produce large light effects at ultrasonicfrequencies, that is substantially independent of frequency, and that isnevertheless attended by little background noise.

A further object of the invention is to provide a new and improvedsystem employing the lightmodulation principles described in the saidcopending application.

Other and further objects will be explained hereinafter and will be moreparticularly pointed out in the appended claims.

The invention will now be more fully described in connection with theaccompanying drawings, in which Fig. 1 is a diagrammatic view ofcircuits and apparatus illustrating the ultrasonic lightmodulationmethod and system disclosed in the said application; Fig. 2 is anexplanatory diagram; Fig. 3 is a reproduction of a photograph obtainedby employing the light-modulation system of Fig. 1, illustrating theefiect produced upon a polarized beam or bundle of light in response tolongitudinal compressional ultrasound waves imparted to a transparentsolid medium through which the beam or bundle is passed; Fig. 4 is areproduction of a similar photograph illustrating the effect oftransverse shearing ultrasound waves, the light beam, however, being ofdifferent polarization; Fig. 5 is a reproduction of a similar photographillustrating the effect produced by employing a medium the transversedimension of which is comparable to the wavelength of the ultrasoundwaves in the medium; Fig. 6 is a diagrammatic view, similar to Fig. l,of a transmitting system constructed in accordance with a preferredembodiment of the present invention; Fig. 7 is a similar view of areceiving system; Fig. 8 is a view similar to Fig. 6 of a modifiedtransmitting system; and Fig. 9 is 2 similarly a view similar to Fig. 7of a modified receiving system.

As disclosed in the said application, a light source I, such, forexample, as a mercury arc, is provided to produce high-intensity lightrays. The present invention is not however restricted to use withvisible light. On the contrary. it has particular use in connection withinfra-red rays. An infra-red or even an ultra-violet filter 3 may.therefore, if desired. be employed. As explained in the saidapplication, a filter 3, adapted to produce monochromatic visible lightof any desired wavelength may also be employed. though the invention isoperable with chromatic as well as monochromatic light.

The light waves are shown collimated by a lens 5 into a parallel beam orbundle of rays of crossdimension corresponding to the cross-dimension ofthe lens 5. The beam or bundle of light rays is caused to pass through aplane polarizer I such. for example, as a Nicol prism or a piece ofpolaroid, and thereafter to impinge upon a substantial area of the frontsurface 9 of a medium I3. The medium I3. of course, should betransparent to the light rays employed, whether visible, infrared orultraviolet, along the direction of travel of the light through themedium l3 between the front surface 9 and the preferably parallel rearsurface II. The transparent medium I3 is preferably of the samecross-dimension as the crossdimension of the light beam. Any otherwellknown focusing system, such as a parabolic reflector, may be used todirect the rays upon the medium I3. The rear surface H of thetransparent medium I3 is shown separated from the front surface 9 by athickness T. The transparent medium I3 may be constituted of a glass ornon-crystalline fused quartz block, or any other transparent solid orliquid. It may or may not be piezoelectric. For the present, it will beassumed. in the further description. that it is not piezoelectric but,on the contrary, that it is optically inactive, birefringent-free. andstrain-free.

The medium l3 may be vibrated molecularly in any desired way. Accordingto the illustrated embodiment of the invention, the vibrations areproduced by means of an ultrasonic vibrator. For the production ofhigh-frequency ultrasonic waves. the vibrator may be constituted of apiezoelectric crystal l5. as of quartz. but it may also be of themagnetostrictive. magnetomotive or of any other suitable type.

The quartz crystal l5 may be vibrated at a predetermined frequency byconnecting its two electrodes ll and I! to an oscillator 2 l. The

period of vibration of the crystal I5 may be relatively low, say,several hundred kilocycles, more or less, or as high as ten megacycles,more or less. The ultrasonic vibrations of the quartz crystal l5adiacent, for example, the bottom edge of the medium, will thereforebecome transmitted or directed into the medium l3 toward the top edge.The medium i3 may be held in place on the crystal IS in any desired way,as by cement or even by a layer of oil to aid in this transmission ordirection.

Let it be assumed, for the moment, that the quartz crystal vibratesalong its thickness dimension, so that it alternately elongates andcontracts vertically. If the height dimension of the medium I3 is equalto a whole multiple of the wavelength of the ultrasound waves in themedium, standing waves of ultrasonic frequency. as before stated, willtheoretically be set up in the medium between its bottom and top edgesurfaces. It is assumed that the medium I3 is of such a nature that.when it is vibrated molecularly to produce these theoretical standingwaves therein, it becomes birefringent to the light passing therethroughalong the direction of travel of the light through the medium which issubstantially perpendicular to the direction of the ultrasound wavesbetween the bottom and top edges.

As the cross-dimension of the parallel beam or bundle of theplane-polarized light rays impinging upon the front surface 9 of themedium l3 corresponds to the cross-dimension of the lens 5, it is largecompared to the dimension of the standing waves produced in the medium13. This is to be contrasted with the conditions obtaining in theprior-art diffraction methods and in the systems employing frequencysensitive crystalline media or shaped media, as discussed in the saidcopending application.

After passing through the medium l3. and emerging from its rear surfaceII, this large parallel beam or bundle of polarized light is shown inFig. 1 focused by a lens 23 upon a screen 21. The lens 23 and the screen21, of course, may be replaced by some other optical system, such as anocular, a camera, a photocell 22, or any other suitable system forreceiving and indicating the beam of light from the rear surface I I ofthe medium 13.

An analyzer 25 is shown interposed between the lens 23 and the screen21. It may be constituted of a piece of polarizing material oriented atright angles to the orientation of the polarizer I. Under normalconditions, therefore, when the circuit of the oscillator 2! is open,and the crystal l5, therefore, is not vibrating, the polarized lightpassing through the medium I3 will be extinguished by the analyzer 25,to a degree depending only on the effectiveness of the polarizing andanalyzing materials, with the result that the screen 21 will be dark.

The term extinguished, of course, is used herein not only in itsordinary sense, as employed ordinarily in connection with visible light,but also more generally to denote also the more general phenomenon ofblocking any of the light waves employed, whether or not visible.

When, however, the circuit of the oscillator 2| is closed. to render iteffective to vibrate the quartz crystal l5, standing waves of ultrasonicfrequency, as already explained, will be set up in the medium l3,between its bottom and top surfaces. Alternate horizontally disposedequally spaced sectional portions of the medium 13 will becomecompressed and dilated by oppositely phased components of the vibrationwaves, in consequence. These compressed and dilated sectional portionswill be separated by portions of the medium I3 wherethe standing soundwaves in the medium l3 will produce nodes. Corresponding changes in therefractive index will occur in the dilated and compressed portions ofthe medium 13, but the refractive index will remain unchanged at thenodes, since these nodal sections are not vibrating. Therefractive-index changes will occur periodically, in synchronism withthe vibrations of the medium.

It has already been stated that the invention is not restricted to usewith monochromatic light. In order to simplify the explanation, however,it will be assumed, for the present, that the light from the source I isactually monochromatic, of wavelength A.

Considering. for the moment, in the plane of the front surface 9 of themedium l3, any one of the horizontally disposed sectional portions ofthe medium 13, let it be assumed that a change dm has occurred in itsrefractive index along the vertical direction V. and that acorresponding change (in: has occurred in its refractive index along thehorizontal direction H, at right angles thereto. Let it further beassumed, for simplicity, that the plane of polarization of the lightpassing through the polarizer 1, upon reaching the plane of the frontsurface 9, is at 45 degrees to the vertical, as indicated at P in Fig.2. This light, of amplitude E0, may therefore be considered as havingtwo equal in-phase polarized components of amplitude The verticalcomponent Ev is polarized along the vertical direction V. and thehorizontal component Ex is polarized along the horizontal direction H.The instantaneous values of these components, at any time t. may berepresented by the following equations:

where w is the angular frequency of the light.

Since the change dm in the index of refraction alongthe verticaldirection V is different from the change dn: in the index of refractionalong the-horizontal direction H, these two polarized components willsuffer difierent phase shifts durin their passage through the medium I3. These phase shifts may respectively be represented by where r is theratio of the circumference to the diameter of a circle.

The .resultant of these two polarized components, upon emerging from therear surface H of the medium B. will therefore no longer, in general,"be a plane-polarized wave. In general, the

wave will be elllptically polarized, and its components will berespectively represented by It is in this elliptically-polarized formthat the components of the elliptically-polarized waves will passthrough the crossed analyzer 25 on their way to the screen 21.

This operation is therefore not the same as that occurring withoptically active crystals or other crystalline substances as beforedescribed. The operation occurring with the optically active crystalsdepends upon rotating the plane of plane polarization. The operationoccurring with permanently doubly retracting crystalline substancesdepends upon modifying the amount of double refraction. The operation ofthe present invention. on the other hand, depends upon depolarizingplane-polarized waves into elliptically polarized waves.

It has been explained that the analyzer 25 may be so oriented that, whenno ultrasonic vibrations whatever are propagated into the medium l3, thescreen 21 is dark. When the oscillator 2! causes the crystal to set upstanding ultrasonic waves in the medium I3, on the other hand, brightlayers, striations, bands, regions or stripes 2 will appear on thescreen 21. The layers, striations. bands, regions or stripes 4 betweenalternately disposed light layers 2 will remain dark.

With the analyzer adjusted in accordance with the above assumptions, soas to extinguish the light passing through the medium l3 at times whenit is not vibrating, the dark stripes will correspond to the lightpassing through the nonvibrating or nodal portions of the medium IS. Thelight stripes 2, on the other hand, will correspond to the successiveportions of the light beam which have become elliptically polarizedduring passage through the corresponding compressed and dilated portionsor sections of the medium l3.

It is not, of course, essential that the analyzer 25 be so oriented asnormally to extinguish the light passing through the medium l3. Theanalyzer 25 may be so oriented that, under normal conditions, when themedium is not vibrating, the screen 21 shall just be illuminated. Thecompressed and dilated sections of the medium I 3, produced in responseto the vibration of the medium l3, may periodically produce ellipticallypolarized light, the major axis of which is normal to the orientation ofthe analyzer 25, so that most of the light passing through thesesections, when they produce such elliptically polarized light, isextinguished. The light stripes 2 will then correspond to the nodes, andalternately disposed dark stripes 4 to the compressed and dilatedsections of the medium 13, produced in response to the vibration of themedium 13.

Inaccordance with the present invention. therefore, the analyzer 25 maybe adjusted initially so as normally either to extinguish the polarizedlight after its passage through the medium l3, or to permit the light topass onto the screen 21. The analyzer 25 may be adjusted to a degreesuch as initially to produce extinction and such that a slight change inthe analyzing process in one direction, resulting from the action of thebirefringent medium I3 to change the state of polarization of the light,will provide bands or sections 2 of illumination alternately with darkbands or sections upon the screen 21. The analyzer 25 may, on the otherhand, be adjusted to a degree such as initially almost, but not quite,to extinguish the analyzed light on the screen 21 and such that an equalchange in the analyzing process in the opposite direction, resultingfrom the action of the birefringent medium on the light will producedark bands or sections alternately with light sections 2 on the screen21.

Since the light and dark layers 2 and 4 are produced periodically, insynchronisrn with the vibrations of the medium I3, the phenomenon, inreality, is produced stroboscopically. Because the frequency of theultrasonic waves is many times greater than that of the flicker limit ofthe eye. however, the effect upon the observer will be the same asthough the light layers 2 were produced with the aid of continuous lightissuin from the analyzer 25.

The difference (mu-(1m) in the changes of the index of refraction alongthe vertical and horizontal directions is known as the birefringence ofthe medium. It is proportional to the total phase shift suffered by thelight in passing through the compressed and dilated sections of themedium l3. The intensity of illumination of the light stripes 2 isproportional to the square of the sine of half the phase shift bsufiered in passing through the medium I3.

The stronger the vibrations of the sound waves, the greater will be thebirefringence and the greater the intensity of illumination of the lightstripes 2. It is accordingly possible to regulate the light intensity inaccordance with the signal produced by the vibrating quartz l5, asdetermined by the oscillations of the oscillator 21. A linearrelationship has been found to exist between the light intensity and thesignal amplitude produced by the quartz IS.

The operation above described has been upon the assumption that theheight dimension of the medium I3 is equal to a whole multiple of thewavelength of the ultrasonic waves propagated through the medium betweenits bottom and top surfaces. This, however, was for explanatory purposesonly. It has been found that, particularly at the higher frequencies. amedium of any arbitrary dimension may be employed, irrespective ofwhether or not it is a multiple of the wavelength. provided only that itis large with respect to the wavelength of the ultrasound waves. It hasalso been found that such a photoelastic-shutter system is notfrequency-sensitive, and the dimensions need bear no specific rela--tionship to the wavelength of the ultrasound waves.

Though the flat area of the crystal I5 is shown. in Fig. l, assubstantially equal to the cross-sectional area of the medium I3 intowhich it propagates the ultrasound waves, this is not essential. Themedium I3 may have a cross-dimension many times the area of the crystall5, though the birefringence effect resulting from the ultrasound waveswill then not be produced strongly throughout the whole medium. Toproduce the birefringence effect throughout such a large medium, andthereby to obtain an unlimited light area, even several square feet, aplurality of vibrators, as shown in Fig. 8, may be employed in contactwith successive portions of the medium l3. A single large flat areacrystal may also be used. No slits or stops are necessary.

The reproduction in Fig. 3 of an actual photo graph obtained with thesystem of Fig. 1 will show, not merely the alternatehorizontally-disposed light and dark layers or stripes 2 and 4, butrather a square-block-like appearance, produced by the addition ofsimilar vertically disposed light and dark layers or stripes. Theseadditional layers or stripes 2 and 4 are probably to be explained by thefact that, during the vibration, every elongation and contraction of thequartz crystal [5 in the vertical direction is automatically accompaniedby a contraction and an elongation, respectively, in the horizontaldirection. These, of course, are also transmitted into the medium I 3.In addition to the theoretical standing waves described above as set upin the medium [3 between the bottom and top surfaces in the verticaldirection, therefore, standing waves appear to be set up also in thehorizontal direction. Upon these standing waves at right angles to eachother, moreover, there are doubtless superposed standing waves in stillother directions, caused by reflection and other phenomena, the effectsof which are not clearly shown in Fig. 3, though their existence appearsto be betrayed in the photograph reproduced in Fig. 5. The result is notmerely the before-described linear vibration of the medium l3, from topto bottom, but rather at least a two-dimensional vibration.

It is fortunate that, at least in the photograph reproduced in Fig. 3,the standing waves in the other directions do not interfere with theoperation, according to the present invention, in the bottom-topdirection. In this photograph, the standing waves at right angles toeach other in the vertical and the horizontal directions are indicatedas having very nearly equal effects upon the incident light. Since thisincident light was described as polarized by the polarizer I at an angleof 45 degrees to the vertical, as represented at P, these standing wavesappear to produce similar effects upon this type of polarized light,even though the vibrations of the two types of vibrations are notprecisely the same. This may explain the block-like appearance of Fig.3.

With the dimensions and materials used, the changes of refractive indexcaused by the transverse shearing strain have been found to be smallerthan those produced by the longitudinal compressional strain in thevertical direction. The

bright striations Ii of Fig. 4 are therefore not so intense as those inthe case illustrated in Fig. 3. Optimum results were found atsubstantially the 45 degree angle of polarization.

Instead of 45-degree polarization, the light issuing from the polarizer1 may be polarized at any other desired angle. The effect of verticalpolarization, for example, may be studied by considering the change ofthe refractive index resulting from the shearing strain produced in themedium It by its transverse vibrations as decomposed along twodirections at right angles to each other along the plane of the surface9. One of these may be an angle of 45 degrees with respect to thevertical, as may also be indicated at P, Fig. 2, and the other may beindicated at P. The components of the vertically-polarized waves alongthe 45-degree directions P and P, during their passage through themedium [3, will sufier diiferent phase shifts, depending on the changesin the refractive index along the two directions P and P. The nature ofthe compressed and dilated sectional portions and the nodal portions ofthe medium I3 produced by the standing waves, of course, will byunchanged by this change in the angle of polarization of the light wavespassing through the medium. For the reasons already given, therefore,owing to the biaxial birefringence thus produced, light layers willstill appear on the screen 21 corresponding to the sheared sectionalportions of the medium. and these will still be separated by dark layerscorresponding to the nodal sectional portions of the medium I3.

These light layers and dark layers are respectively shown at 6 and 8 onactual photograph. reproduced in Fig. 4, taken when employing light ofvertical polarization in the system of Fig. 1.

It makes a difference, therefore, not only theoretically, but also inpractice, whether the polarization of the incident waves lies in oneplane or another plane. This again demonstrates that the operation,according to the present invention, is not the same as that with anoptically active crystal, the operation of which depends upon rotationof the plane of plane polarization, and not the depolarizing of theplane-polarized waves into elliptically polarized waves.

The distances between the centers of the successively disposed light ordark striations or layers are respectively equal to one-half thewavelength A1 of the longitudinal compressional vibrations and one-halfthe wavelength M of the transverse compressional vibrations propagatedvertically and horizontally, respectively, in the medium 13. The valuesof these wavelengths obtained by measurement may be used directly tofind Young's modulus e and Poissons ratio 0'.

As an example, ultrasound waves of a frequency f=10 megacycles werepropagated, as illustrated in Fig. 1, into a sample [3 of plate glass3.1 x 1.4 x 1.0 centimeters, having a density p=2.61. Upon measurement,the wavelength A: of the longitudinal waves in the vertical directionwas found to be 0.532 millimeter and the wavelength M of the transversewaves was found to be 0.312 millimeter. Young's modulus e was thencalculated to be '7.t5 10 degrees/cm. from the formula 1-0 This valueagrees with the values obtained for the same sample [3 by diffractionmethods.

The ratio m of the longitudinal to the transverse wavelengths,

has been found to be in close agreement with the ratio of thedisplacements of the first diffraction orders produced by the transverseand the longitudinal waves. Poisson's ratio 0 may then be obtained fromthe equation light layers. intersecting patterns and scrolls I werefound, probably resulting from numerous reflections within the medium.

Rough corners or edges have been found to introduce no uncertainties inthe operation.

The invention has heretofore been explained in connection withbirefringent-free and strainfree media l3. It is preferable to employmedia [3 of this character, and to adjust the analyzer 25 so as toobtain complete extinction before the vibrations are initiated in themedium. Extremely intense stroboscopic-light layers have thus beenobserved, for example, with noncrystalline fused-quartz media.

Chrystalline substances having permanent birefringence, however, mayalso theoretically be employed. They also demonstrate light-intensitychanges when subjected to ultrasonic waves, as described above. If anoptically active medium 43. such as a piezo-electn'c crystal. is used.however, then. in order to detect the effect of the birefringence on thelight emerging from the face I l of the medium. it is necessary toorient the analyzer 25 at right angles to the polarizer, and not so asto extinguish the light emerging from the medium, the plane ofpolarization of which has been rotated in passing through the medium. Itis then only that the birefringence effect, in addition to the opticalactivity, may theoretically be detected. In practice. the much strongereffect of the change in optical activity produced by straining themedium may prevent the detection of the effects of birefringence.

The use, in the system of Fig. 1, of a strain free, optically-inactive,non-crystalline, isotropic medium 13 that is normally birefringent-freealong the direction of travel of the light rays between the frontsurface 9 and the rear surface H of the medium l3, therefore, not onlymakes possible the measurements above-described and the observance ofthe effects of ultrasonics in such media, but it also provides a largecontinuous area of stroboscopic light which truly flashes from darknessto light of a high intensity. In the case of one crown-glass samplemedium I3 of about the same dimensions as the sample previouslydiscussed, the intensity change was found to be equal to one-tenth ofthe intensity of the mercury arc itself. Illumination of this order ofintensity can be used, for example. to photograph scientific or otherphenomena in motion at ultrasonic or other high frequencies. Thefrequency of the oscillator 2| needs merely to be adjusted until themoving object appears to stand still.

If a quarter-wave plate is inserted in front of each polarizing device 1and 25, so that circularlypolarized. instead of plane-polarized, lightis impinged upon the medium l3, and is analyzed after emergingtherefrom, even stronger-intensity results occur. While some of theintensity resulting from the birefringence produced by the longitudinalwaves is lost. this is apparently more than made up for by the lightintensity resulting from the birefringence produced by the transversewaves. Waves having initially elliptical polarization may also beemployed.

Continous-wave signals may be transmitted by the system of Fig. l or ofFig. 6 in many ways, as by interrupting the circuit of the oscillator 2|with a key. Modulated audio or video signals may be transmitted bymodulating the carrier frequency of the oscillator 2|, as by means of amodulator 20. Since the thin high-frequency crystal I5 may be vibratedwith wide side-bandwidths, the vibrations of the quartz crystal l5,oscillating at a high carrier frequency while in contact with the medium13, are correspondingly modulated in accordance with the audio or videosignal. The non-resonant, relatively largedimensioned medium [3 has beenfound to respond sufiiciently instantaneously to the modulatedultrasonic carrier, propagated thereinto from the crystal IE. to producebirefringence in response to the modulation signal.

At the receiving station. the lens 23 may therefore be caused to focusthe birefringenceproduced elliptically-polarized light from the mediuml3 on to some other light-receiving means than the screen 21. Asillustrated in Fig. 7, for example. a photocell 22 may be employed toreceive elliptically polarized light. In order to detect the modulation,the photocell 22 may be connected to an audio or video amplifier 28.Crystalline media I: may also be used for the photocell or photographicdetection of this lightmodulation transmission, though with limitationsof frequency sensitiveness, and accompanied by background noise orlight.

The photocell 22 is itself sensitive to only a band of wavelengths. Bydesigning it so that it shall respond with maximum effect to lighthaving the wavelength of infrared or other monochromatic rays, mostsensitive and most noiseless operation may then be obtained withmonochromatic light of such wavelength.

It has been found possible to transmit as many as three differentmodulated signals through the system of Fig. 6 simultaneously, with goodreproduction. at the receiving end, of all three signals. Thetransmission may be effected by feeding one or more of the modulatingsignals to the same quartz or other ultrasonic vibrator 15. Suchprocedure tends, however, to overload the mechanical parts of thesystem. The multi channels may also be obtained by employing a pluralityof vibrators l5 and I5, as shown in Fig. 8, in sonic contact with themedium, and modulating the vibrators with different signals from themodulators 20 and 20. If the different modulations are of entirelydifferent frequency ranges, such as correspond, for example. to an audiomodulation and a video modulation, there will be very little distortionwhen received in the receiving system of Fig. 9 which is operated in asimilar manner to the operation of the receiving system of Fig. 7. Thephotocell may feed tuned amplifiers. such as the amplifiers 2B and 28'which will separate out and amplify the intelligence of the differentchannels; for example, the amplifier 28 may separate out and amplify theaudio channel while the amplifier 28' amplifies the video channel.

Audio frequencies ranging from 40 cycles to 15,000 cycles have been usedto modulate a ten-megacycle ultrasonic carrier with noiseless anddistortionless results.

The above-described system finds particular application also intelevision-projection systems such, for example, as the Scophony system,where liquid diffraction cells have heretofore been employed.

To control the volume and the performance of this light-modulationsystem, it is desirable: first. properly to orient the plane ofpolarization of the polarizer; secondly, to control the operation of thepiezo-electric carrier on and off the resonant frequency of the-piezo-electric crystal; thirdly; to position the medium l3 so as tointercept more or less of the incident light; and fourthly, suitably toinsert or remove a diaphragm, or to control the aperture of a diaphragmin the path of the light beam.

As a modification, if plane-polarized light is used, for example, theanalyzer 25 may be provided with suitable phase-shifting plates. Theelliptically polarized waves emerging from the compressed and dilatedregions of the medium l3 will thus be properly analyzed, whil permittingthe plane-polarized light passing through the nodal-region portions ofthe medium to penetrate the analyzer 25. The use of a quarter-wave platewith the analyzer 25 may be particularly desirable, for example, wherethe thickness T of the medium is such as to produce exactly aninety-degree phase shift between the components V and H of the incidentlight, circularly polarizing the light emerging from the compressed anddilated regions.

The description above has been simplified on the assumption thatmonochromatic light is used. Monochromatic light has its practicalapplications. The signalling system of Fig. 6, for example, could beused with infra-red or other invisible rays to provide added secrecy.The discussion above is equally applicable, however. to all wavelengths,even when employed simultaneously, that is, to chromatic light.

Further modifications will occur to persons skilled in the art, and allsuch are considered to fall within the spirit and scope of the inventionas defined in the appended claims.

What is claimed is:

1. A communication system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined vibrational wavelength toproduce standing waves therein between a pair of edges of the mediumspaced from each other in a direction substantially perpendicular to thesaid predetermined direction a distance corresponding to several timesthe said wavelength, becomes birefringent to the light passingtherethrough along the predetermined direction, means for passingthrough the medium along the predetermined direction a bundle of lightof cross-dimension large compared to the wavelength of the standingwaves, a polarizer for polarizing the light prior to its pas sagethrough the medium, an analyzer for analyzing the light after itspassage through the medium, the analyzer being adjusted so that a changein the analyzing produced thereby in one direction will provideextinction of the analyzed light and a substantially equal change in theanalyzing produced thereby in the opposite direction will permit thepassage of the analyzed light, means for roducing molecular vibrationsof the said predetermined wavelength at one of the said edges of themedium, means for directing the vibrations into the medium toward theother said edge of the medium in order that the vibrations may bereflected therefrom to set up the said standing waves, means for varyingthe molecular vibrations, and means for receiving the analyzed light todetect the variation.

2. Apparatus as set forth in claim 1 the vibrating means of whichcomprises means for propagating ultrasonic waves into the medium.

3. Apparatus as set forth in claim 1 the'vibrating means of whichcomprises piezo-electric means.

4. Apparatus as set forth in claim 1 all of the dimensions of the mediumof which are large 12 compared to the wave-length of the standing waves.

5. A communication system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined vibrational wavelength toproduce standing waves therein between a pair of edges of the mediumspaced from each other in a direction substantially perpendicular to thesaid predetermined direction a distance corresponding to several timesthe said wavelength, becomes birefringent to the light passingtherethrough along the predetermined direction, means for passingthrough the medium along the predetermined direction a bundle of lightof cross-dimension large compared to the wavelength of the standingwaves, a polarizer for polarizing the light prior to its passage throughthe medium, an analyzer for extinguishing the light after its passagethrough the medium, means for producing molecular vibrations of th saidpredetermined wavelength at one of the said edges of the medium. meansfor directing the vibrations into the medium toward the other said edgeof the medium in order that the vibrations may be reflected therefrom toset up the said standing waves. means for modulating the molecularvibrations, and means for receiving the analyzed light to detect themodulations.

6. Apparatus as set forth in claim 5 the vibrating means of whichcomprises means for pr pagating ultrasonic waves into the medium.

A communication system having, in comination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined vibrational wavelength toproduce standing waves therein between a pair of edges of the mediumspaced from each other in a direction substantially perpendicular to thesaid predetermined direction a distance corresponding to several timesthe said wavelength, becomes birefringent to th light passingtherethrough along the predetermined direction, means for passingthrough the medium along the predetermined direction a bundle of lightof cross-dimension large compared to the wavelength of the standingwaves, a polarizer for polarizing the light prior to its passage throughthe medium, an analyzer for extinguishing the light after its passagethrough the medium, means for producing molecular vibrations of the saidpredetermined wavelength at one of the said edges of the medium, meansfor directing the vibrations into the medium toward the other said edgeof the medium in order that the vibrations may be reflected therefrom toset up the said standing waves. means for modulating the molecularvibrations with a plurality of independent sigfl'afl'f'fifefiii sforreceiving the analyzed light,;and me'afifcoopFa e tive with thereceivingf eansfor sepiTratfig'aiid amplifying the independent signals.

8.Acoiis m amg,incom bination, a medium that is transparent to lightalong a predetermined direction and that, when molecularly vibrated at apredetermined ultrasonic wavelength to produce standing waves thereinbetween a pair of edges of the medium spaced from each other in adirection substantially perpendicular to the said predetermineddirection a distance corresponding to several times the said wavelength,becomes birefringent to the light passing therethrough along thepredetermined direction, means for passing through u u i.- HUM-m 13 themedium along the predetermined direction a bundle of light ofcross-dimension large compared to the wavelength of the standing waves,a polarizer for polarizing the light prior to its passag through themedium, an analyzer for analyzing the light after its passage throughthe medium, the analyzer being adjusted so that the analyzed light isalmost but not quite extinguished, means for producing molecularvibrations of the said predetermined wavelength at one of the said edgesof the medium, means for directing the vibrations into the medium towardthe other said edge of the medium in order that the vibrations may bereflected therefrom to set up the said standing waves, means formodulating the molecular vibrations with an audio signal, and means forreceiving the analyzed light to detect the signal.

9. A transmitter having, in combination, a medium that is transparent tolight along a predetermined direction and that, when molecularlyvibrated at a predetermined vibrational wavelength to produce standingwaves therein between a pair of edges of the medium spaced from eachother in a direction substantially perpendicular to the saidpredetermined direction a distance corresponding to several times thsaid wavelength, becomes birefringent to light passing therethroughalong the predetermined direction, means for passing light through themedium along the predetermined direction, a polarizer for polarizing thlight prior to its passage through the medium, means for producingmolecular vibrations of the said predetermined wavelength at one of thesaid dges of the medium, means for directing the vibrations into themedium toward the other said edge of the medium in order that thevibrations may be reflected therefrom to set up the said standing waves,and means for modulating the molecular vibrations of the medium.

10. A communication system having, in combination, a medium having frontand rear surfaces and that is transparent to light along the directionbetween the front and rear surfaces,

the medium being normally birefringent-free to light propagated alongthe said direction, means for passing a bundle of light toward asubstantial area of the front surface, a polarizer interposed in thepath of the light, an analyzer positioned in the path of the lightemerging from the rear surface after its passage through the medium,means for producing ultrasonic vibrations at an edge of the medium ofwavelength small compared with the distance between the said edge and asecond edge of the medium, means for directing the vibrations into themedium toward the said second edge in order that the vibrations may bereflected therefrom to set up standing waves that render th mediumbirefringent to the light, means for modulating the molecular vibrationswith an audio signal, and means positioned beyond the analyzer forrendering th signal detectable.

11. A communication system having, in combination, a medium having frontand rear surfaces and that is transparent to light, means for passing abundle of light toward a substantial area of the front surface, a planepolarizer interposed in the path of the light and oriented along adimension of the medium, an analyzer positioned in the path of the lightemerging from the rear surface after its passage through the medium andoriented at right angles to the polarizer, means for producingcarrier-wave vibrations at an edge of the medium of wavelength smallcompared with the distance between the said edge and a second edge ofthe medium from which the Vibrations may be reflected, means formodulating the amplitude of the said carrier-wave vibrations, and meanspositioned beyond the analyzer for rendering the modulations detectable.

12. A communication system having, in combination, a medium having frontand rear surfaces and that is transparent to light, means for passing abundle of light toward a substantial area of the front surface, a planepolarizer interposed in the path of the light and oriented at forty-fivedegrees to a dimension of the medium. an analyzer positioned in the pathof the light emerging from the rear surface after its passage throughthe medium and oriented at right angles to the polarizer, means forproducing carrierwave vibrations at an edge of the medium of wavelengthsmall compared with the distance between the said edge and a second edgeof the medium from which the vibrations may be reflected, means formodulating the amplitude of the said carrier-wave vibrations, and meanspositioned beyond the analyzer for rendering the modulations detectable.

13. A communication system having, in combination, a medium having frontand rear surfaces and that is transparent to light, means for passing abundle of light toward a substantial area of the front surface, meansinterposed in the path of the bundle of light for circularly polarizingthe light, means positioned in the path of the light emerging from therear surface after its passage through the medium for analyzing thelight, means for producing carrier-wave vibrations at an edge of themedium of wavelength small compared with the distance between the saidedge and a second edge of the medium from which the vibrations may bereflected, means for varying a property of the wave, and meanspositioned beyond the analyzer for rendering the variation detectable.

14. A communication system having, in combination, a medium havingsubstantially parallel front and rear surfaces, that is transparent tolight. and that is of a predetermined refractive index, means forpassing a bundle of light toward a substantial area of the frontsurface, means for polarizing the light prior to its reaching the frontsurface, means oriented normally to the polarizing means for analyzingthe light emerging from the rear surface after its passage through themedium, means for periodically straining the medium at equally spacedintervals along a direction substantially parallel to the front and rearsurfaces in order to alter the refractive index of the medium at thesaid intervals by a predetermined amount along one directionsubstantially parallel to the said surfaces and by a diflferent amountalong a direction at an angle to the said one direction, thereby torender the medium periodically birefringent, means for varying theperiodic birefringence in accordance with a signal, and means positionedbeyond the analyzing means for rendering the signal detectable.

15. Apparatus as set forth in claim 14 the medium of which is initiallystrain-free and the straining means of which comprises means forpropagating ultrasound waves into the medium.

16. A communication system having, in com bination, a medium havingsubstantially parallel front and rear surfaces separated by a thicknessT. that is transparent to light, and that is of refractive index 11,means for passing a bundle of light having a predetermined wavelength 1toward a substantial area of the front surface, means forplane-polarizing the light prior to its reaching the front surface,means for analyzing the light emerging from the rear surface after itspassage through the medium in the direction of the thickness '1, meansfor periodically straining the medium at equally spaced intervals alonga direction substantially parallel to the front and rear surfaces inorder to alter the refractive index of the medium at the said intervalsby an amount dni along one direction at an angle to the plane ofpolarization of the light and by a different amaunt dm along a seconddirection at right angles to the said one direction, thereby to shiftthe phase of the component of the polarized light substantially parallelto the said one direction by an amount and -to shift the phase of thecomponent of the polarized light substantially parallel to the saidsecond direction by an amount whereby the light emerging from the rearsurface of the medium becomes periodically converted into ellipticallypolarized light prior to its becoming analyzed by the analyzing means,and means for altering the periodic straining in accordance with asignal of lower periodicity.

17. A communication system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at vibrational wavelengths less than apredetermined vibrational wavelength to produce standing waves thereinbetween a pair of edges of the medium spaced from each other in adirection substantially perpendicular to the said predetermineddirection a distance corresponding to several times the said wavelength,becomes birefringent to the light passing therethrough along thepredetermined direction, means for passing through the medium along thepredetermined direction a bundle of light of cross-dimension largecompared to the dimension of the standing waves, a polarizer forpolarizing the light prior to its passage through the medium, ananalyzer for extinguishing the light after its passage through themedium, a plurality of means disposed adjacent one of the said edges ofthe medium for producing molecular vibrations of the said vibrationalwavelengths and for directing the vibrations into the medium toward thesaid other edge of the medium in order that the vibrations may bereflected therefrom to set up the said standing waves, means connectedto the vibrating means for modulating the molecular vibrations, andmeans for receiving the analyzed light to detect the modulations.

18. A communication system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at vibrational wavelengths less than apredetermined vibrational wavelength to produce standing waves thereinbetween a pair of edges of the medium spaced from each other in adirection substantially perpendicular to the said predetermineddirection a distance corresponding to several times the said wavelength,becomes birefringent to the light passing therethrough along thepredetermined direction, means for passing through the medium along thepredetermined direction a bundle of light of cross-dimension largecompared to the dimension of the standing waves, a polarizer forpolarizing the light prior to its passage through the medium, ananalyzer for extinguishing the light after its passage through themedium, a plurality of piezoelectric crystals disposed adjacent one ofthe said edges of the medium for producing molecular vibrations of thesaid vibrational wavelengths and for directing the vibrations into themedium toward the said other edge of the medium in order that thevibrations may be reflected therefrom to set up the said standing waves,means connected to the piezoelectric crystals for modulating themolecular vibrations, and means 10: receiving the analyzed light todetect the modulations.

19. A transmitter having, in combination, a medium that is transparentto light along a Predetermined direction and that, when molecularlyvibrated at vibrational wavelengths less than a predeterminedvibrational wavelength to produce standing waves therein between a pairof edges or tne medium spaced from each other in a directionsubstantially prependicular to the said predetermined direction adistance corresponding to severaltimes the said wavelength, becomesbirefringent to light passing therethrough along the predetermineddirection, means for passing light through the medium along thepredetermined direction, a polarizer for polarizing the light prior toits passage through the medium, a plurality of means disposed adjacentone of the said edges of the medium for producing molecular vibrationsof the said vibrational wavelengths and for directing the vibrationsinto the medium toward the said other edge of the medium in order thatthe vibrations may be reflected therefrom to set up the said standingwaves, and means connected to the vibrating means for modulating themolecular vibrations of the medium.

20. A transmitting system having, in combination, a. medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined vibrational wavelength toproduce standing waves therein between a pair of edges of the mediumspaced from each other in a direction substantially perpendicular to thesaid predetermined directlon a distance corresponding to several timesthe said wavelength. becomes birefringent to the light passingtherethrough along the predetermined direction, means for passingthrough a plurality of successive portions of the medium disposedbetween the said edges. each of width substantially equal to thehalf-wavelength of the standing waves, along the predetermineddirection, a beam of light having a plurality of successive portionscorresponding to the plurality of successive half-wavelength portions ofthe medium, plane-polarizing means disposed to pclarize the successivebeam portions prior to their passage through the corresponding mediumportions, means for producing molecular vibrations of the saidpredetermined wavelength at one of the said edges of the medium, meansfor directing the vibrations into the medium toward the other said edgeof the medium in order that the vibrations may be reflected therefrom toset up the said standing waves, thereby rendering the successive mediumportions birefringent in antiphase in response to the resulting standingwaves,

17 and means for modifying the vibrations in accordance with a signal.

21. A transmitting system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined ultrasonic vibrationalwavelength to produce standing waves therein between a pair of edges ofthe medium spaced from each other in a direction substantiallyperpendicular to the said predetermined direction a distancecorresponding to several times the said wavelength, becomes birefringentto the light passing therethrough along the predetermined direction,means for passing through a plurality of successive portions of themedium disposed between the said edges, each of width substantiallyequal to the halfwavelength of the standing waves, along thepredetermined direction, a beam of light having a plurality ofsuccessive portions corresponding to the plurality of successivehalf-wavelength portions of the medium, plane-polarizing means disposedto polarize the successive beam portions prior to their passage throughthe corresponding medium portions, means for producing molecularvibrations of the said predetermined wavelength at one of the said edgesof the medium, means for directing the vibrations into the medium towardthe other said edge of the medium in order that the vibrations may bereflected therefrom to set up the said standing waves, thereby renderingthe successive medium portions birefringent in anti-phase in response tothe resulting standing waves, and mean for modifying the vibrations inaccordance with a signal of greater wavelength.

22. A transmitting system having, in combination, a medium that istransparent to light along a predetermined direction and that. whenmolecularly vibrated at a predetermined vibrational wavelength toproduce standing waves therein between a pair of edges of the mediumspaced from each other in a direction substantially perpendicular to thesaid predetermined direction a distance corresponding to several timesthe said wavelength becomes birefringent to the light passingtherethrough along the predetermined direction, means for passingthrough a plurality of successive portions of the medium disposedbetween the said edges, each of width substan tially equal to thehalf-wavelength of the standing waves, along the predetermineddirection, a beam of light having a plurality of successive portionscorresponding to the plurality of successive half-wavelength portions ofthe medium, plane-polarizing means disposed to polarize the successivebeam portions prior to their passage through the corresponding mediumportions at forty-five degrees with respect to the direction ofpropagation of the standing waves, means for' producing molecularvibrations oi. the said predetermined wavelength at one of the saidedges of the medium, means for directing the vibrations into the mediumtoward the other said edge of the medium in order that the vibrationsmay be reflected therefrom to set up the said standing waves, therebyrendering the successive medium portions birefringent in anti-phase inresponse to the resulting standing waves, and means for modifying thevibrations in accordance with a signal.

23. A transmitting system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined vibrational wavelength toproduce standing waves therein between a pair of edges of the mediumspaced from each other in a. direction substantially perpendicular tothe said predetermined direction a distance corresponding to severaltimes the said wavelength, becomes birefringent to the light passingtherethrough along the predetermined direction, means for passingthrough a plurality of successive portions of the medium disposedbetween the said edges. each of width substantially equal to thehalf-wavelength of the standing waves. along the predetermineddirection, a beam of light having a plurality of successive portionscorresponding to the plurality of successive half-wavelengh portions ofthe medium, plane-polarizing means disposed to polarize the successivebeam portions prior to their passage through the corresponding mediumportions in a plane parallel to the said predetermined direction, meansfor producing molecular vibrations of the said predetermined wavelengthat one of the said edges of the medium, means for directing thevibrations into the medium toward the other said edge of the medium inorder that the vibrations may be reflected therefrom to set up the saidstanding waves, thereby rendering the successive medium portionsbirefringent in antiphase in response to the resulting standing waves,and means for modifying the vibrations in accordance with a signal.

24. A transmitting system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined vibrational wavelength toproduce standing waves therein between a pair or edges of the mediumspaced from each other in a direction substantially perpendicular to thesaid predetermined direction a distance corresponding to several timesthe said wavelength, becomes birefringent to the light passingtherethrough along the predetermined direction, means for passingthrough a plurality of successive portions of the medium disposedbetween the said edges. each of width substantially equal to thehalf-wavelength of the standing waves, along the predetermineddirection, a beam of light having a plurality oi successive portionscorresponding to the plurality of successive half-wavelength portions ofthe medium, circular-polarinng means disposed to polarize the successivebeam portions prior to their passage through the correspondlng mediumportions. means for producing molecular vibrations of the saidpredetermined wavelength at one of the said edges of the medium. meansfor directing the vibrations into the medium toward the other said edgeof the medium in order that the vibrations may be reflected therefrom toset up the said standing waves, thereby rendering the successive mediumportions birefringent in antiphase in response to the resulting standingwaves, and means for modifying the vibrations in accordance with asignal.

25. A communication system having, in combination, a medium that istransparent to light along a predetermined direction and that, whenmolecularly vibrated at a predetermined ultrasonic carrier vibrationalwavelength to produce standing waves therein between a pair of edges ofthe medium spaced from each other in a direction substantiallyperpendicular to the said predetermined direction a distancecorresponding to several times the said wavelength, becomes bir f n t0he li h passing therethrough along the predetermined direction, meansfor determined direction, a beam of light having a plurality ofsuccessive portions correspondnig to the plurality oi successivehalf-wavelength portions of the medium. plane-polarizing means disposedto polarize the successive beam ortions prior to their passage throughthe corresponding medium portions, means for producing molecularvibrations of the said predetermined wavelength at one of the said edgesof the medium, means for directing the vibrations into the medium towardthe other said edge oi the medium in order that the vibrations may bereflected therefrom to set up the said standing waves. thereby renderingthe successive medium portions birefringent in anti-phase, means formodulating the amplitude of the carrier-wave vibrations in accordancewith a signal of greater wavelength, means for receiving the lightpassed through the medium, means for converting the received light intoplanepolarized light, and means for detecting the modulation of greaterwavelength upon the ultrasonic carrier wavelength as variations in theconverted light to reproduce the signal of greater wavelength.

EANS MUELLER. ROBERT E. RINES.

namnmvcns crran The iollowing references are oi. record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,565,566 Hartley Dec. 15, 19251,642,011 Chubb Sept. 13, 1927 1,694,661 Meissner Dec. 11, 19281,742,912 Hartley Jan. 7, 1930 1,849,488 Hanna Mar. 15. 1932 1,880,102Meissner Sept. 27, 1932 1,885,604 Karolus Nov. 1, 1932 1,954,947 PajesApr. 17, 1934 1,997,371 Loiseau Apr. 9, 1935 1,997,628 Chubb Apr. 16,1935 2,084,201 Karolus June 15, 1937 2,099,694 Land Nov. 23, 19372,234,329 Wolfi Mar. 11, 1941 2,418,964 Arenberg Apr. 15, 1947 2,531,951Shamos et a1 Nov. 28, 1950 FOREIGN rii'rmws Number Country Date 132,858Great Britain Sept. 23, 1919

